Polyimide laminate and a method of fabricating the same

ABSTRACT

Disclosed herein are a polyimide laminate and a method for fabricating the same. In the disclosed method, a polyimide film having a thermally-conductive filler distributed homogenously therein is prepared, the polyimide film is characterized in having a thermal conductivity greater than 0.3 W/m-° C. Then, at least one metal film is subsequently deposited on one or both sides of the polyimide film by electroplating, electroless plating, evaporation, sputtering or lamination and thereby forming the desired polyimide laminate.

RELATED APPLICATIONS

This application claims priority to Taiwan Application Serial Number97147890, filed Dec. 9, 2008, which is herein incorporated by reference.

BACKGROUND

1. Field of Invention

The present disclosure in general relates to a circuit board and amethod to of fabricating the same. More particularly, the presentdisclosure is related to a polyimide laminate and a method offabricating the same.

2. Description of Related Art

With the prevalence of flexible circuit boards (FCPs) over the knownprinted circuit boards (PCB) in electronic communication devices, FCPsmarket has enjoyed a rapid growth in recent years. Further, being lightweighted and easy to carry also contribute the growing popularity ofFCPs in meeting product demands of end applications.

A flexible circuit board laminate is usually composed of a plasticsubstrate, and a plurality of metal layers disposed thereon, whereineach of the metal layers includes wires for connecting other circuitdevices. Heat is commonly generated in a typical metal film producingprocess, if not dissipated properly, would accumulated on the substrateand eventually deteriorates the substrate, particularly, the flexiblesubstrate.

In view of the above, there exists in this art a need of an improvedflexible substrate, which exhibits improved thermal conductivity and mayredirect the heat generated during the manufacturing process to otherheat-dissipating elements so as to reduce the temperature to a desiredlevel. The improved flexible substrate of the present disclosure may beapplied to the manufacturing process of metal films.

SUMMARY

In view of the above, the objective of this disclosure aims to providean improved polyimide laminate and a method of fabricating the same.

In the first aspect, the disclosure provides a modified of fabricating apolyimide laminate. The method includes steps of: forming a polyimidefilm having a thermally conductive filler distributed homogeneouslytherein with the thermally conductive filler being about 10-90% byweight of the polyimide solid is and thereby rendering the polyimidefilm having a thermal conductivity greater than 0.3 W/m-° C.; andforming at least one metal layer on one side of the polyimide film.

The at least one metal layer is made of any of Pd, Cu, Ni, Fe Al, or acombination thereof.

The thermally conductive filler has a thermal conductivity greater than10 W/m-° C. and is any of a metal oxide, a metal nitride, carbon,silicon carbide or ceramic powders. The metal oxide may be aluminumoxide, and the metal nitride may be any of aluminum nitride, boronnitride, a sintered form thereof, or a combination thereof.

The at least one metal layer may be formed by electroplating,electroless plating, sputter deposition, vapor deposition or lamination.

In one example, a layer of Pd formed by electroless plating and a layerof Cu formed by electroplating are deposited in sequence on one side ofthe polyimide film.

In another example, a layer of Ni and a layer of Cu respectively formedby sputter deposition, and another layer of Cu formed by electroplatingare deposited in sequence on one side of the polyimide film.

In still another example, a layer of Ni formed by vapor deposition, anda layer of Cu formed by electroplating are deposited in sequence on oneside of the polyimide film.

In still another example, a layer of Cu is laminated on one side of thepolyimide film.

In another embodiment of the disclosed method, at least one metal layeris respectively deposited on each side of the polyimide film. In oneexample, a layer of Pd formed by electroless plating and a layer of Cuformed by electroplating are respectively deposited in sequence on eachside of the polyimide film. In another example, a layer of Ni and alayer of Cu respectively formed by sputter deposition, and another layerof Cu formed by electroplating are respectively deposited in sequence oneach side of the polyimide film. In still another example, a layer of Niformed by vapor deposition, and a layer of Cu formed by electroplatingare respectively deposited in sequence on each side of the polyimidefilm. In still another example, a layer of Cu is laminated on each sideof the polyimide film.

In a second aspect of this disclosure, a polyimide laminate having atleast one metal layer deposited on one or two sides of a polyimide filmis provided. The polyimide film has a thermal conductivity of greaterthan 0.3 W/m-° C.

In one embodiment, the polyimide laminate has at least one metal layerdeposited on one side of the polyimide film. In one example, thepolyimide laminate includes a layer of Pd and a layer of Cu deposited insequence on one side of the polyimide film. In another example, thepolyimide laminate includes a layer of Ni, a first layer of Cu and asecond layer of Cu deposited in sequence on one side of the polyimidefilm. In still another example, the polyimide laminate includes a layerof Ni and a layer of Cu deposited in sequence on one side of thepolyimide film. In still another example, the polyimide laminateincludes a layer of Cu laminated on one side of the polyimide film.

In another embodiment, the polyimide laminate has at least one metallayer respectively deposited on each side of the polyimide film. In oneexample, each side of the polyimide laminate includes a layer of Pd anda layer of Cu deposited in sequence on the polyimide film. In anotherexample, each side of the polyimide laminate includes a layer of Ni, afirst layer of Cu and a second layer of Cu deposited in sequence on thepolyimide film. In still another example, each side of the polyimidelaminate includes a layer of Ni and a layer of Cu deposited in sequenceon the polyimide film. In still another example, each side of thepolyimide laminate includes a layer of Cu laminated on the polyimidefilm.

These and other features, aspects, and advantages of the presentdisclosure will become better understood with reference to the followingdescription and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated in and constitutea part of this specification. The drawings illustrate embodiments of thedisclosure and, together with the description, serve to explain theprinciples of the disclosure. In the drawings,

FIG. 1 is a schematic diagram illustrating an apparatus for fabricatingthe polyimide film in according to one embodiment of this disclosure.

DETAIL DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

Embodiments of the present disclosure are directed to a method offabricating a polyimide laminate. The method includes steps of: forminga polyimide film having a thermally conductive filler distributedhomogeneously therein with the thermally conductive filler being about10-90% by weight of the polyimide solid and thereby rendering thepolyimide film having a thermal conductivity greater than 0.3 W/m-° C.;and forming at least one metal layer on one side of the polyimide film.

FIG. 1 is a schematic diagram illustrating an apparatus 100 forperforming the method of this disclosure. A polyamic acid solution 120is prepared in a reactor 110 of the apparatus 100. The polyamic acidsolution 120 may be made by any suitable procedures, for example, one ofthe reactants for producing polyamic acid 124, the aromatic diamine, isdissolved in a solvent 126, followed by the addition of the otherreactant, aromatic tetracarboxylic dianhydride, and a thermallyconductive filler 122. The two reactants in the mixture, the aromaticdiamine and the aromatic tetracarboxylic dianhydride, are allowed toreact and form polyamic acid 124, with the thermally conductive filler122 being homogeneously distributed within the mixture of the polyamicacid 124 and the solvent 126.

The method of making the polyamic acid solution 120 or polyamic acid 124has been disclosed in a published Taiwan patent application No: TW200914502 filed by the same applicant on Sep. 29, 2007 and published onApr. 1, 2009, the disclosure thereof is incorporated herein byreference.

Suitable aromatic diamine for use in the disclosed method includes, butis not limited to, 1,4-diaminobenezene, 1,3-diaminobenezene,4,4′-oxydianiline, 3,4′-oxydianiline, 4,4′-methylene dianiline,N,N′-diphenylethylenediamine, diaminobenzophenone, diaminodiaphenylsulfone, 1,5-naphthalene diamine, 4,4′-diaminodiphenyl sulfide,1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene,1,3-bis(4-aminophenoxy)benzene,2,2-bis[4-(4-aminophenoxy)phenoxy]propane,4,4′-bis(4-aminophenoxy)biphenyl, 4,4′-bis(3-aminophenoxy)biphenyl,1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane,1,3-bis(3-aminopropyl)-1,1,3,3-tetraphenyldisiloxane,1,3-bis(aminopropyl)-dimethyldiphenyldisiloxane or a combinationthereof. Suitable aromatic tetracarboxylic dianhydride for use in thedisclosed method includes, but is not limited to, 1,2,4,5-benzenetetracarboxylic dianhydride, 3,3′,4,4′-biphenyl tetracarboxylicdianhydride, 4,4′-oxydiphthalic anhydride, benzophenone tetracarboxylicdianhydride, 3,3′,4,4′-biphenyl sulfone tetracarboxylic dianhydride,1,2,5,6-nathalene tetracarboxylic dianhydride, Naphthalenetetracarboxylic dianhydride, Bis(3,4-dicarboxyphenyl)dimethylsilanedianhydride, 1,3-bis(3,4-phthalic anhydride)-tetramethyl disiloxane or acombination thereof.

The aromatic diamine and the aromatic tetracarboxylic dianhydride may beused in a molar ratio of between about 1.1:1 to about 0.9:1.

Suitable solvent that may be used in the present disclosure includes,but is not limited to, N,N-dimethyl formamide (DMF), dimethyl acetamide(DMAc), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), or acombination thereof.

Suitable thermally conductive filler that may be employed in thedisclosed method may be an inorganic material having a thermalconductivity greater than 10 W/m° C., and may be any of a metal oxidesuch as aluminum oxide; a metal nitride such as aluminum nitride, boronnitride, a sintered form thereof or a combination thereof; a carbonmaterial such as carbon, carbon black, graphite or a carbon nanotube;silicon carbide; ceramic powders or a combination thereof. In oneexample, aluminum oxide is employed as the thermally conductive filler.The thermally conductive filler is used in an amount of about 10-90% byweight of the polyimide solid, such as 10, 20, 30, 40, 50, 60, 70, 80,or 90% by weight of the polyimide solid.

The polyamic acid solution 120 thus prepared may be optionally stored ina reservoir 130 until further use. In the process of producing thepolyimide film, a constant amount of the polyamic acid solution 120 isdelivered continuously from the reservoir 130 to a delivered head 140,while a steel strip 150 is continuously fed through an inlet 162 of afilm-producing device 160 by a driving means 170 and acts as a carrierfor carrying the constant amount of the polyamic acid solution 120delivered from the delivered head 140 to the steel strip 150 forproducing the polyimide film. The driving means includes a wheel 172 anda supporting cylinder 174.

The delivered head 140 may be a head suitable for blade coating, slotcoating, or extrusion coating. The polyamic acid solution 120 may bedriven by weight or by pressure and is delivered continuously in aconstant amount from the head 140 to the surface of the rotating steelstrip 150 driven by the driving means 170. The head 140 is set to beseparated from the surface of the rotating steel strip 150 by a distance“d”, which is about 60-1500 μm, so as to allow the polyamic acidsolution 120 in various thickness to be held on the surface of the steelstrip 150 and subsequently form polyimide films in various is thickness.By controlling the distance “d” or the pressure for delivering thepolyamic acid solution 120, the purpose of forming polyamic acidsolution 120 in various thicknesses may be achieved easily.

The polyamic acid solution 120 that is held on the surface of the steelstrip 150 is subsequently heated to form polyimide film thereon. Inoperation, the steel strip 150 carried thereon the polyamic acidsolution is driven by the driving means 170 and passes the heatingdevice 180, so that the polyamic acid solution may be heated at atemperature between 80-400° C. and thereby forms the polyimide film 190,which is subsequently outputted from the outlet 164.

The outputted polyimide film 190 may then be used in a metallizationprocess to fabricate the desired polyimide laminate.

The metallization process may be performed on either one side or bothsides of the polyimide film 190 fabricated by the method describedabove. At least one metal layer is deposited on either one or both sidesof the polyimide film 190 by any of a method, which includes but is notlimited to, electroplating, electroless plating, sputter deposition,vapor deposition, or lamination. The thickness of each metal layer maybe adjusted by the specification of the intended application and anyperson having ordinary skill in the related art may adjust the processparameters to determine the most suitable condition for depositing eachmetal layer without undue experimentation.

Suitable metal for use in the metallization process includes, but is notlimited to, Pd, Cu, Al, Ni, Fe, or a combination thereof.

In one embodiment, the metallization process is performed on only oneside of the polyimide film 190, In one example, a layer of Pd formed byelectroless plating and a layer of Cu formed by electroplating aredeposited in sequence on one side of the polyimide film. In anotherexample, a layer of Ni and a layer of Cu respectively formed by sputterdeposition, and another layer of Cu formed by electroplating aredeposited in sequence on one side of the polyimide film. In stillanother example, a layer of Ni formed by vapor deposition, and a layerof Cu formed by electroplating are deposited in sequence on one side ofthe polyimide film. In still another example, a layer of Cu is laminatedon one side of the polyimide film.

In another embodiment of the disclosed method, the metallization processis performed on both sides of the polyimide film 190, that is, at leastone metal layer is respectively deposited on each side of the polyimidefilm. In one example, each side of the polyimide film includes insequence: a layer of Pd formed by electroless plating and a layer of Cuformed by electroplating. In another example, each side of the polyimidefilm includes in sequence: a layer of Ni and a layer of Cu respectivelyformed by sputter deposition, and another layer of Cu formed byelectroplating. In still another example, each side of the polyimidefilm includes in sequence: a layer of Ni formed by vapor deposition, anda layer of Cu formed by electroplating. In still another example, alayer of Cu is laminated on each side of the polyimide film.

Example The Fabrication of a Polyimide Laminate 1. The Preparation of aPolyimide Film

Suitable amounts of p-phenylene diamine and diamino diphenyl ether asindicated in Table I were added in N-methyl pyrrolidone; then aluminumoxide was added and the mixture was continuously stirred for 1 hour.1,2,4,5-benzene tetracarboxylic dianhydride and 3,3′,4,4′-biphenyltetracarboxylic dianhydride were slowly added to the solution and themixture was stirred for 6 hours to form the polyamic acid solution. Thepolyamic acid solution was then delivered continuously in a constantamount to the surface of the steel strip 150 of the apparatus 100 asdepicted in FIG. 1, and heated at a temperature of about 80-400° C.under an atmosphere of nitrogen to form a polyimide film with athickness about 25 μm. The polyimide film was then taken out from thesteel strip 150 after its temperature returned to room temperature. Thepolyimide film of examples 1 and 2 were about 19.23% and 19.85% byweight of the polyamic acid solid; and the thermal conductivity, waterabsorption activity and electrical property of the polyimide film ofexamples 1 and 2 are provided in Table II.

Comparative Examples 1 and 2

A polyimide film without a thermally-conductive filler distributedtherein was manufactured by use of the ingredients indicated in Table Iin according to the steps described above, except no aluminum oxide wasincluded. The polyimide film of comparative examples 1 and 2 were about16% and 19.85% by weight of the polyamic acid solid; and the thermalconductivity, water absorption activity and electrical property of thepolyimide film of comparative examples 1 and 2 are provided in Table II.

TABLE I Comparative Comparative Example 1 Example 2 Example 1 Example 2p-phenylene diamine (g) 8.94 8.67 8.94 8.67 diamino diphenyl ether (g)6.62 6.42 6.62 6.42 N-methyl pyrrolidone (g) 252 252 252 252 Aluminumoxide (g) 12 14.4 0 0 1,2,4,5-benzene 3.57 3.83 3.57 3.83tetracarboxylic dianhydride (g) 3,3′,4,4′-biphenyl 28.88 29.07 28.8829.07 tetracarboxylic dianhydride (g) Polyimide (% of 19.23 19.85 1619.85 polyamic acid solid)

TABLE II Comparative Comparative Example 1 Example 2 Example 1 Example 2Thermal conductivity (W/m-° C.) 0.5 0.6 0.17 0.17 Water absorptionactivity (%) 2.1 1.7 2.8 3.2 Bulk Resistance (Ω cm) 10¹³ 10¹³ 10¹³ 10¹³Surface Resistance (Ω) 10¹³ 10¹³ 10¹³ 10¹³ Breakdown Voltage (KV) 5.54.5 6 5.8

It is clear from Tables I and II, with the addition of thermallyconductive fillers such as aluminum oxide in the polyamic acid solution,the thermal conductivity of the resulted polyimide film would increasefrom about 0.17 W/m-° C. to about 0.5 or 0.6 W/m-° C. In other words,the thermal conductivity of the polyimide film increased with theinclusion of thermally conductive fillers there within.

Further, the water absorption activity of the polyimide film decreasedwith the inclusion of aluminum oxide there within the film, therebyrendering the film exhibiting better dielectric activity that compliedwith the specification of a high frequency circuit board.

As to the electrical property of the polyimide film, it remainedrelatively the same with or without the addition of aluminum oxide, withthe bulk and surface resistance being 10¹³Ω and 10¹³ Ω·cm, respectively.Further, the break down voltage also remained at the level of about 2kV.

2. The Preparation of Polyimide Laminate 2.1 Forming Metal Layers onPolyimide Film of Example 1 or 2 by Electroplating

The polyimide film of Example 1 or 2 was first cleaned with a washingsolution containing permangant ions, and then with a reducing agent. Thecleaned polyimide film of Example 1 or 2 was then immersed in apreparative solution, which contained polymeric electrolytes such aspolyethylene imidazole that contains a tetra-valence metal ion.

The polyimide film of Example 1 or 2 was first plated with a layer ofprecious metal (such as Pd) by immersion the polyimide film in aelectrolyte containing therein a precious metal ion gel and a reducingagent. The precious metal ion gel is a gel containing palladium ions.The reducing agent may be ascorbic acid, biphenyl, hydroxylamine or aderivative thereof, or formaldehyde. The polyimide film of Example 1 or2 was placed in a cell containing therein the precious metal ion gel,the reducing agent was then poured in to start the electroless platingprocedure, and a thin layer of Pd was respectively formed on each sideof the polyimide film of Example 1 or 2.

This polyimide film having a thin layer of Pd respectively formed oneach side was then placed in an electroplating cell for plating a thinlayer of Cu on each side. Alternatively, a thin layer of copper may beelectroplated on only one side of the polyimide film. The electrolytefor copper plating was copper sulfate or copper pyrophosphate, and theelectrolyte solution was mildly stirred during plating. The plated Culayer was about 50 μm in thickness. It is to be noted that in generalthe plating solution for industrial use usually contains leveling agent,gloss-enhancer or the like, which may cause damage to the polyimidefilm, hence may not be used in this invention.

After the Cu plating was completed, the thus obtained polyimide laminatewas then subjected to etching process to form desired circuit patternthereon, and a final layer of Au was formed on top of the circuitpattern to protect the circuit pattern from oxidation.

2.2 Forming Metal Layers on Polyimide Film of Example 1 or 2 by SputterDeposition and Electroplating

The polyimide film of Example 1 or 2 was pre-cleaned in a vacuum chamberby corona discharge or plasma etching. Then, a thin layer of Ni wassputter deposited on either one or both sides of the polyimide film witha thickness being controlled to be in the range of about 50 Å to 500 Å;followed by sputter deposition a layer of Cu thereon. The Cu layer maybe formed on either one or both sides of the polyimide film.

Then, another layer of Cu was electroplated in accordance with theprocedures described above in section 2.1. Similarly, the layer of Cumay be electroplated on either one or both sides of the polyimide filmwith a thickness of about 50 μm. The thus formed polyimide laminate wasthen proceeded to form desired circuit pattern and an outer most layerof Au according to steps described in section 2.1.

2.3 Forming Metal Layers on Polyimide Film of Example 1 or 2 by VaporDeposition

A layer of Ni was formed on either one or both sides of the polyimidefilm of example 1 or 2 by vapor deposition. The thickness of the Nilayer was less than 500 Å, and preferably between 50 Å to 300 Å.Alternatively, Fe or Al may be used in place of Ni.

Then, a layer of Cu was electroplated thereon in accordance with thesteps described in section 2.1. Similarly, the Cu layer may beelectroplated on either one or both sides of the polyimide film. Thethus formed polyimide laminate was then subjected to etching process toform desired circuit pattern thereon and an outer most Au layer toprotect the circuit pattern from oxidation.

2.4 Forming Metal Layers on Polyimide Film of Example 1 or 2 byLamination

The polyimide film of example 1 or 2 was laminated with a layer of Cu inaccordance with known procedures for lamination. The layer of Cu may beformed by electroplating or by rolling milling with a thickness betweenabout 5 μm to 50 μm, preferably between about 5 μm to 35 μm, and asmooth surface. The layer of Cu may be replaced by a layer of Al or Fe.The thus formed polyimide laminate may be stored in rolls until furtheruses.

INDUSTRIAL APPLICATION

The present disclosure provides an improved method of fabricating apolyimide laminate. The polyimide laminate includes a polyimide filmhaving thermally conductive filler homogeneously distributed therein,and at least one metal layer formed on either one or both sides of thepolyimide film. The thermally conductive filler homogeneouslydistributed within the polyimide film renders the film with good thermalconductivity and therefore may prevent excess heat generated during anysubsequent process from being accumulated within the polyimide film andthereby would prevent deformation of the film. Hence, the polyimidelaminate of the present disclosure is suitable for use as a flexiblesubstrate in semiconductor applications.

The foregoing description of various embodiments of the disclosure hasbeen presented for purpose of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the preciseembodiments disclosed. Numerous modifications or variations are possiblein light of the above teachings. The embodiments discussed were chosenand described to provide the best illustration of the principles of thedisclosure and its practical application to thereby enable one ofordinary skill in the art to utilize the disclosure in variousembodiments and with various modifications as are suited to theparticular use contemplated. All such modifications and variations arewithin the scope of the disclosure as determined by the appended claimswhen interpreted in accordance with the breadth to which they arefairly, legally, and equitably entitled.

1. A method of fabricating a polyimide laminate, comprising: forming apolyimide film having a thermally conductive filler distributedhomogeneously therein with the thermally conductive filler being about10-90% by weight of the polyimide solid and thereby rendering thepolyimide film having a thermal conductivity greater than 0.3 W/m-° C.;and forming at least one metal layer on one side of the polyimide film.2. The method of claim 1, wherein the at least one metal layer comprisesa metal that is selected from a group consisting of Pd, Cu, Al, Fe, Niand a combination thereof.
 3. The method of claim 2, wherein the atleast one metal layer is formed by electroplating, electroless plating,sputter deposition, vapor deposition or lamination on one side of thepolyimide film.
 4. The method of claim 3, wherein a layer of Pd formedby electroless plating and a layer of Cu formed by electroplating aredeposited in sequence on one side of the polyimide film.
 5. The methodof claim 3, wherein a layer of Ni and a layer of Cu respectively formedby sputter deposition, and another layer of Cu formed by electroplatingare deposited in sequence on one side of the polyimide film.
 6. Themethod of claim 3, wherein a layer of Ni formed by vapor deposition, anda layer of Cu formed by electroplating are deposited in sequence on oneside of the polyimide film.
 7. The method of claim 3, wherein a layer ofCu is laminated on one side of the polyimide film.
 8. The method ofclaim 2, further comprising forming the at least one metal layer on theother side of the polyimide film.
 9. The method of claim 8, wherein theat least one metal layer is formed by electroplating, electrolessplating, sputter deposition, vapor deposition or lamination on the otherside of the polyimide film.
 10. The method of claim 9, wherein a layerof Pd formed by electroless plating and a layer of Cu formed byelectroplating are respectively deposited in sequence on both sides ofthe polyimide film.
 11. The method of claim 9, wherein a layer of Ni anda layer of Cu respectively formed by sputter deposition, and anotherlayer of Cu formed by electroplating are respectively deposited insequence on both sides of the polyimide film.
 12. The method of claim 9,wherein a layer of Ni formed by vapor deposition, and a layer of Cuformed by electroplating are respectively deposited in sequence on bothsides of the polyimide film.
 13. The method of claim 9, wherein a layerof Cu is laminated respectively on each side of the polyimide film. 14.The method of claim 1, wherein the thermally conductive filler has athermal conductivity of greater than 10 W/m-° C. and is any of metaloxide, metal nitride, carbon, silicon carbide (SiC) or ceramic powders.15. The method of claim 14, wherein the metal oxide is aluminum oxide.16. The method of claim 14, wherein the metal nitride is any of aluminumnitride, boron nitride, a sintered form thereof or a combinationthereof.
 17. A polyimide laminate produced by the method of claim
 3. 18.A polyimide laminate produced by the method of claim 9.